Dendrites are microscopic crystalline structures that form between parallel conductors, for example, on a printed circuit board surface between two copper traces that have different voltage potentials. The dendrites develop over a long period of time and may eventually extend from trace-to-trace. These structures are electrically conductive, and when extending from trace to trace form a short circuit. Dendrite structures are caused by the voltage difference acting on surface contaminants on the printed circuit board. A more detailed discussion of dendrite is found in, Metal Migration in Encapsulated Modules and Time-to-Fail Model as a Function of the Environment and Package Properties, Givlio DiGiacomo, IEEE Proceedings, 1982, incorporated herein by reference.
Dendrites, or other materials, can create a parallel parasitic resistance in various electronic devices, causing short circuits or other malfunctions under certain operating parameters. For example, in a wireless security system, sensors communicate with a system controller via RF-transmitted message packets. Each sensor includes a transmitter (typically an integrated circuit or chip) that is connected to other circuitry on a small printed circuit board. Although located in an enclosed package, the sensor circuit board is exposed to environmental contaminants, and dendrites or other materials may accumulate on the sensor circuit board.
Additionally, dendrites or other parasitic parallel resistance agents may form in other areas of the sensor circuit. For example, dendrite-induced short circuits may develop within the sensor. For example, window screen sensors are susceptible to the development of parallel parasitic resistances. Parasitic parallel resistances may also develop between the external contacts of the transmitter.
During operation, conditions exist that are conducive to dendrites forming short circuits on the sensor circuit board and at remotely wired detector sensors. These short circuits may appear to the transmitter as a change in condition, when no actual change in condition has occurred. The transmitter then sends incorrect information to the system controller. Therefore, there is a need for preventing or eliminating dendrite short circuits on the sensor circuit board and at remotely wired detector sensors. Additionally, there is a need to eliminate or overcome other types of parasitic parallel resistances within the sensor system.
Implementing a short circuit reduction feature must be balanced with other important design considerations in a wireless security system. The sensors of the wireless security system are typically battery powered. For convenience and reliability purposes, the sensor and transmitter circuitry are designed to minimize energy consumption and extend the battery life. Additionally, certifying organizations, such as Underwriters Laboratories, require minimum periods for which the batteries must power the sensor.
In order to meet the low energy consumption goal, the battery-powered sensor and transmitter systems are typically operated a low currents. These low-current operating conditions are conducive to dendrite-induced short circuits and other types of parasitic parallel resistance malfunctions. Operating the circuits at higher currents would tend to reduce the parasitic parallel resistance problems, but also reduce battery life. Therefore, there is a conflict in the need to extend battery life and to prevent malfunctions induced by parasitic parallel resistance.